An interfacial polycondensation method and a transesterification method are known as a process for producing a polycarbonate. From the standpoint of quality, an interfacial polycondensation method using a halogen-containing organic solvent such as methylene chloride is currently the main trend.
In the interfacial polycondensation method, a polycarbonate is obtained by reacting a dihydric phenol with phosgene. In this case, a process in which an oligomer is first produced and is thereafter further subjected to polycondensation to produce a polycarbonate is generally adopted from the standpoint of reaction control, etc.
When a polycarbonate is produced by such an interfacial polycondensation method, a mixture of a polycarbonate-containing organic solvent solution and an aqueous solution is obtained after termination of the polycondensation. Since the polycarbonate-containing organic solvent solution is generally in the form of an emulsion, it is necessary to separate the emulsion into an organic solvent solution of the polycarbonate and an aqueous solution containing impurities.
In general, as a method for efficiently carrying out liquid-liquid separation, a method utilizing a centrifuge to forcibly apply a gravity force is known. With this method, apparatus costs become high although the separation efficiency is good. Further, since an interface is substantially present in the centrifuge separator, an intermediate phase accumulates therein. Thus, a problem that the process must be periodically stopped for cleaning still remains even if the accumulation is reduced.
The aqueous solution containing impurities becomes a wastewater. For the removal of phenol compounds contained in the wastewater, a method is known in which such a wastewater is contacted with an oligomer to transfer the unreacted phenol to a polycarbonate oligomer solution, the thus obtained unreacted phenol-containing polycarbonate oligomer solution being thereafter subjected to a polymerization treatment (Patent Document 1). Also, a method in which a polycarbonate oligomer is produced under specific conditions for the purpose of obtaining a polycarbonate having favorable properties and a wastewater having a high purity is known (Patent Document 2).
With regard to the separation into an organic solvent solution containing the polycarbonate oligomer and an aqueous solution after the above-described treatment, however, these publications merely disclose a still standing separator method.
In order to obtain a wastewater having desired properties in Patent Document 1 and Patent Document 2, not only the treatment conditions in the process but also the separation method is important. When the separation is insufficient, not only phenol compounds but also a part of the organic solvent solution containing a polycarbonate oligomer contaminates the wastewater. As a consequence, a wastewater which is sufficiently clean cannot be obtained.
Since the phenol compounds contained in an emulsion solution of a polycarbonate oligomer-containing organic solvent contain hydrophilic hydroxide groups and lipophilic aromatic rings, they can function as a surfactant. Therefore, it is difficult to separate them. Further, in general, when two liquids having different specific gravities are separated by still standing, an intermediate layer which is recognized to be unavoidably formed accumulates. Therefore, it is often necessary to periodically conduct cleaning or, as the case may be, to stop the process for conducting cleaning.
Additionally, when water containing impurities remains in the separated polycarbonate oligomer, the quality of the polycarbonate is deteriorated so that it is difficult to obtain a resin having a high heat resistance from such polycarbonate.
On the other hand, it is widely known to use a coalescer for liquid-liquid separation. Thus, various techniques have been disclosed for the liquid-liquid separation using coalescers in polycarbonate production processes (Patent Documents 3 to 7).
These techniques, however, are directed to the separation of a polymer solution mixture obtained after the termination of the polycarbonate polymerization but are not related to separation of a polycarbonate oligomer solution mixture or separation under the conditions where a large amount of phenol compounds are present in an aqueous phase.
Patent Document 1: Japanese Laid Open Patent Publication No. H08-245780
Patent Document 2: Japanese Laid Open Patent Publication No. 2002-338679
Patent Document 3: Japanese Patent Publication No. S46-41622
Patent Document 4: Japanese Laid Open Patent Publication No. S55-104316
Patent Document 5: Japanese Laid Open Patent Publication No. H07-309940
Patent Document 6: Japanese Laid Open Patent Publication No. 2001-31755
Patent Document 7: Japanese Publication of Translation of PCT No. 2002-528580